1. Field of the Invention
The present invention relates to an electronic pressure-sensitive device for detecting the magnitude of a load as electrical resistance and, in particular, to an electronic pressure-sensitive device capable of detecting the magnitude of a load ranging from a small load to a large load.
2. Description of the Related Art
For example, as shown in FIG. 7, in a conventional electronic pressure-sensitive device used in a seat sensor or the like detecting whether a person is sitting on a seat, a first contact portion 52 formed on a flexible first insulating substrate 51 and consisting of a conductive material is opposed to a second contact portion 56 formed on a flexible second insulating substrate 55 and consisting of a conductive material through the intermediation a gap, the width of the gap being maintained by a spacer 54 provided between the first and second insulating substrates.
When a load is applied to the first and second insulating substrates 51 and 55, the insulating substrates 51 and 55 are deflected toward the gap. And, the load applied to the first and second insulating substrates 51 and 55 exceeds a threshold value, the first contact portion 52 is brought into contact with the second contact portion 56, and the electronic pressure-sensitive device is turned ON.
In the conventional seat sensor, a plurality of electronic pressure sensitive devices as described above are arranged in the plane of a seat base (the seat base receiving the weight of a person), and, from the in-plane distribution of the electronic pressure-sensitive device which has been turned ON, it is detected whether a person is sitting on the seat or not. And, when the presence of a person sitting on the seat is detected, an air bag is operated when the vehicle undergoes a collision.
However, in the conventional electronic pressure-sensitive device, which is turned ON when a load of a magnitude not less than a threshold value is applied, it is difficult to detect a small load applied to the electronic pressure-sensitive device or variation in the load.
Further, since the first and second insulating substrates 51 and 55 are deflected toward the gap, the burden applied to the first and second insulating substrates 51 and 55 is large, and, when the first and second insulating substrates 51 and 55 are deformed so as to protrude toward the gap, the threshold value varies to thereby cause malfunction.
In a motor-vehicle-mounted seat sensor using this conventional electronic pressure-sensitive device, it is difficult to detect the magnitude of a load, so that it is impossible to sense the physique of the person sitting on the seat, to operate the air bag in conformity with the physique of the person sitting on the seat, some other device is necessary. Further, in the case of a motor-vehicle-mounted sensor, a load can be continuously applied at high temperature, so that it is necessary to use an expensive material such as polyethylene terephthalate, which has a superior heat resistance, for the first and second insulating substrates 51 and 55, which are continuously deflected by the load (baggage), resulting in a high cost.
Accordingly, it is an object of the present invention to provide an electronic pressure-sensitive device which has a long service life and which is capable of detecting loads of various magnitudes including a minute load in appropriate resistance values.
In accordance with the present invention, there is provided an electronic pressure-sensitive device comprising a first substrate consisting of an insulating material, a first contact portion formed on the first substrate, a first resistive element provided on the surface of the first contact portion, a second substrate consisting of an insulating material and opposed to the first substrate, a second contact portion formed on the second substrate and opposed to the first substrate, a high resistance material, a second resistive element constituting the first contact portion and provided on the surface of the second contact portion, and an adhesive member provided between the first and second substrates and adapted to glue the first and second substrates to each other, wherein the first and second resistive elements are constantly in an electrical contact state, and wherein a press contact force acting between the first and second resistive elements is detected as the electrical resistance between the first and second contact portions by pressurizing the first and second contact portions from at least one side.
In this electronic pressure-sensitive device, the first and second resistive elements consisting of a high resistance material are constantly in an electrical contact state, so that, even when no load is applied to the substrates, the electrical resistance measured between the first and second contact portions is a limited value. When a load is applied to at least one of the first and second substrates, even if it is a minute load, the pressure contact force which acts between the first and second resistive elements according to the load increases, and the electrical contact area of the first and second resistive elements increases, whereby the contact resistance decreases, so that the electrical resistance measured between the first and second contact portions is a value according to the load. Thus, due to the electrical resistance measured between the first and second contact portions, it is possible to detect loads of various magnitudes including those in the proximity of zero.
Further, since there is no need to maintain a gap between the contact portions, there is no need to provide a maintaining member for maintaining a gap, and there is no deterioration in performance due to deformation of the maintaining member. Further, since there is no gap between the contact portions, the amount of deformation (deflection amount) due to the load is small, so that there is little burden on the substrates, thereby providing a long service life.
Further, even when the magnitudes of the press contact forces acting on the first and second resistive elements are the same, the higher the specific resistance of the first and second resistive element materials, the higher the electrical resistance measured between the first and second contact portions, so that, by varying the specific resistance of the first and second resistive element materials according to the magnitude of the load detected by the electronic pressure-sensitive device (the specifications of the electronic pressure-sensitive device), the electrical resistance measured between the first and second contact portions is an appropriate value, making it possible to detect the load according to the specifications in an appropriate electrical resistance.
Further, by gluing together the first and second substrates by an adhesive member, the first and second resistive elements are secured at predetermined positions, and the opposition area of the first and second resistive elements is maintained constant, so that it is possible to reduce the factor leading variation in the detection of the load.
Further, in the electronic pressure-sensitive device of the present invention, the adhesive member is formed at a position deviated from the first and second resistive elements in such a way as to surround the first and second resistive elements.
In this electronic pressure-sensitive device, the adhesive member surrounds the first and second resistive elements with the first and second substrates to make it possible to reduce the influence from the external environment. Further, the electrical contact of the first and second resistive elements is not hindered by the adhesive member.
Further, in the electronic pressure-sensitive device of the present invention, at least one of the first and second substrates is flexible, and the adhesive member consists of a sheet-like member, the gap between the first and second substrates being larger at the positions where the first and second contact portions are formed than at the position where the adhesive member is provided.
In this electronic pressure-sensitive device, the substrates are deflected between the adhesive member and the contact portions, so that the first and second contact portions are pressurized from the side of the deflected substrates, whereby a press contact force acts on the first and second resistive elements, making it possible to constantly keep the first and second resistive elements in an electrical contact state without imparting any external force.
Further, when the sum total of the thicknesses of the first and second contact portions is the same and no external force (load) is applied, the press contact force (referred to as pressurization) acting on the first and second contact portions (resistive elements) due to the deflected substrates is larger when the adhesive member is thin than when it is thick.
Thus, even when the magnitude of the load applied to the substrates is the same, the electrical resistance between the first and second contact portions is lower when the adhesive member is thin than when it is thick. However, when the load applied to the substrates is sufficiently large, and the press contact force (pressurization) due to the deflection of the substrates is negligible, the electrical resistance between the first and second contact portions converges on a value which does not depend on the thickness of the adhesive member. Thus, the variation in the electrical resistance between the contact portions with respect to the variation in the load applied to the substrates is milder when the adhesive member is thin.
Utilizing the above load/electrical-resistance characteristics, the thickness of the adhesive member is varied according to the width between the minimum value and the maximum value of the load detected by the electronic pressure-sensitive device (the specifications of the electronic pressure sensitive device), whereby adjustment can be conducted from the electrical resistance corresponding to the minimum load to the electrical resistance corresponding to the maximum load, so that it is possible to detect a load within a desired detection range in an appropriate electrical resistance.
Further, in the electronic pressure-sensitive device of the present invention, both the first and second substrates are flexible, so that it has a superior shock resistance and the substrates are not damaged by the load or the like.
Further, in the electronic pressure-sensitive device of the present invention, an opening is provided in the adhesive member consisting of a sheet-like member, and the first and second resistive elements are brought into an electrical contact state inside this opening, and the sum total of the thicknesses of the first and second contact portions is larger than the distance between the first and second substrates at the position where the adhesive member is provided.
Thus, it is possible to bring the first and second resistive elements exposed through the opening due to the thicknesses of the first and second contact portions and the thickness of the sheet-like member even in the case in which no external force is applied.
Further, in the electronic pressure-sensitive device of the present invention, the adhesive member is formed such that adhesive layers are formed on both sides of the sheet-like member, so that there is little variation in the thickness of the adhesive member, and it is possible to produce the device such that the distance between the first and second substrates is constant.
Further, in the electronic pressure-sensitive device of the present invention, at least one of the first and second contact portions has a conductor consisting of a conductive material electrically connected to the first and second resistive elements.
In this electronic pressure-sensitive device, it is possible to effect electrical connection between the first and second resistive elements and the exterior through the conductor, so that it is possible to effect more reliably an electrical connection between the first and second resistive elements and the exterior.
Further, in the electronic pressure-sensitive device of the present invention, the first and second contact portions have the first and second conductors, respectively, and the first and second conductors are opposed to each other through the first and second resistive elements, and formed in the entire area where the first and second resistive elements are opposed to each other.
In this electronic pressure-sensitive device, the current path between the first and second conductors is substantially perpendicular to the opposing surfaces of the first and second resistive elements, and is the shortest path in the first and second resistive elements, so that, on either side of the electrical contact surfaces of the first and second resistive elements, the electrical resistance of the current path can be made low. Thus, the electrical resistance output between the first and second contact portions can be such that the component due to the contact resistance is predominant.
Further, in substantially the entire area of the opposing surfaces of the first and second resistive elements, current paths are distributed, so that in the resistant output, the variation in the electrical characteristics in the first and second resistive element surfaces is averaged, giving a stable output.
Further, in the electronic pressure-sensitive device of the present invention, the first and second resistive elements are respectively formed on the surfaces of the first and second conductors, and the surface area of the resistive elements is different from the surface area of the conductors.
In this electronic pressure-sensitive device, the surface area of the conductors and that of the resistive elements are different, so that in the process for superimposing the resistive elements on the conductors, misregistration in formation between them (e.g., misregisration in printing) can be absorbed due to the difference in magnitude as compared with the case in which the surface area of the conductors and that of the resistive elements are the same, so that it is easy to make the overlapping area of the resistive elements and the conductors, making it possible restrain variation in characteristics.
Further, in the electronic pressure-sensitive device of the present invention, the resistive elements cover the entire surface of the conductors.
In this electronic pressure-sensitive device, the conductors are not exposed to the exterior, so that it is possible to prevent short-circuiting between the conductors and the other contact portions. Further, since the conductors are cut off from the external environment, it is possible to prevent a deterioration in the conductors.
Further, in the electronic pressure-sensitive device of the present invention, the first and second resistive elements are formed of the same material, so that it is possible to reduce the number of kinds of resistive element material, whereby it is easy to control the materials and it is possible to prevent generation of improper use.
Further, in the electronic pressure-sensitive device of the present invention, the first and second resistive elements are formed of a material whose specific resistance ranges from 102 to 106 (xcexa9xc2x7cm), so that the resistance value measured between the first and second contact portions is an appropriate value, making it possible to correctly detect the load.
To perform the detection more accurately, it is desirable that the specific resistance of the material range from 103 to 105 (xcexa9xc2x7cm).
Further, in the electronic pressure-sensitive device of the present invention, a wiring pattern consisting of a conductive material is formed on at least one of the first and second substrates, and the wiring pattern is electrically connected to the first and second resistive elements.
In this electronic pressure-sensitive device, power supply from the outside to the contact portions and the resistance value output between the first and second contact points (power output in accordance with the resistance value) are effected by using the wiring pattern, so that there is no need to separately connect a lead wire or the like to the resistive elements, so that the construction is simple, and it is possible to make the electrical connection between the contact portions and the exterior more reliable.